Piston and method of making the same



Dec. 17, 1935. z, JEFFRIES ET AL I 2,024,767 I P ISTON AND METHOD OF MAKING THE SAME Filed June 22, 1952 flaw; 14 [(EMPE I Patented Dec. 17, 1935 UNITED STATES PISTON AND METHOD OF MAKING THE SAME Zay Jefirles, and Louis W. Kempf, Cleveland Ohio, assignors to Aluminum Company of America, Pittsburgh, Pa", a corporation of Pennsylvania Application June 22, 1932, Serial No. 618,644

7 Claims.

Our invention relates to the manufacture of pistons for internal combustion engines. It is particularly directed to a new process for fabricating such pistons, and to the entirely novel product of this process. I

Since the development of molds andmethods whereby light alloy'pistons could be successfully cast in chill molds, there has been an increasing trend in the industry towards the use of these light weight pistons until at present millions are made and used every year. Of the light metals, aluminum base alloys have been most" widely used up to the present time. For the purposes of this invention it is unnecessary to describe in detail the advantages of light alloy pistons which mainly have their'source in low specific gravity and high heat conductivity. This invention is more particularly concerned with improvement attempt to meet this difilculty.

of light alloy pistons in certain respects wherein their potentialities have not been fully developed heretofore.

Under the conditions usually obtaining in a tomotive engines, an aluminum base alloy piston reciprocates' within a ferrous metal cylinder. Because of the difference in the coefiicients of thermal expansion of the cylinder and piston, if the piston is fitted closely when the motor is cold, it will seize at operating temperatures, and if it is to be designed with suflicient clearance so that it will fit properly at operating temperatures the clearance when cold is so great that piston slap is experienced when starting and during the period necessary to bring the parts to working temperature.

Various means have so far been devised in an One llllQ'Of development has been to slit or otherwise design the skirt of the piston to give a pliable bearing surface calculated to flt snugly at all temperatures. wherein a ferrous metal insert is integrally cast or otherwise connected to the light metal skirt to restrain its expansion so as to approximate the expansion of the ferrous metal cylinder. A multitude of these and similar devices have already been furnished to the trade, some of but little value and some with a fair measure of success. However, the straight trunk type piston,

namely, a piston with a cylindrical skirt unslotted and integral with the head, is considered most desirable if only the expansion differential can be reduced or eliminated. I

The most successful attempt heretofore made, so far as we are advised; to produce a low expansion light alloy for pistons is disclosed, inU,

Others have devised control methods-f 8. Patent No. 1,799,837 to R. S.,-Archer and L. W. Kempf. 'I'his,patent discloses a series of alloys, of which the preferred composition has a coeflicient of thermal expansion of .000019 per. degree centigrade." The previous preferred aluminumbase alloy for pistons had a c'oefilcient of expansion of .000022 and since the coefiicient of expansion of cast ironis .000011 it is seen that the alloy of U. 8. Patent No. 1,799,837 has reduced the diiierential by about 27%. Because 10 of complications resulting from the inclusion of constituents calculated to lower the thermal expansiomfurther improvements in this direction are handicapped by operative dificulties, for instance increasing casting and machining trou-- bles. As a consequence while it is desirable to still further decrease the expansion, it has been economically impracticable to fabricate the pistons until the date of the present invention.

Although the advantages of employing compositions having a low coefficient of thermal expansion are obvious and although many attempts have been made to obtain such compositions in usable form, no success has been had in decreas-. ing the coefficient of thermal expansion below about .000019 per degree centigrade, when using a light alloy base in making pistons.

We have discovered that the problem of obtaining a satisfactory coefllcient of expansion with a light alloy base can be. solved by. avoiding cast ing methods and by' using a method in which comminuted metals are compressed to .form either the finished article or a blank from which the'flnished article maybe machined. The (iii-"- ficulties connected with casting may thus be chin 3:, inated and a wide range of compositions is made available which heretofore has not proved-successful by any. casting method. Thus certain alloying constituents which serve to lowerfth e.

slum base alloys may successfully be and a coeflicient of thermal expansion attained which has not heretofore been approximated with-light alloy cast pistons. I

Accordingly, one of the objects of thelpresent invention is to devise a. method whereby light metal pistons of a" wide variety of compositions may be successfully fabricated,

Afurther object of this invention is the provision of a method of making a piston of low specific gravity. I

A further object of this invention is thepro vision of a method of making a light alloy. piston of exceptionally low thermal expansivity.

This invention is further concerned with a piston of low specific gravity, exceptionally low coeflicient of thermal expansion, high heat 'conductivity and good wearing qualities fabricated by a process novel in the art.

Other objects and advantages will become apparent from the hereinbelow description of the invention and from the attached drawing wherein Figure 1 is an elevation with parts broken away to illustrate the piston blank as initially formed according to my invention. Figure 2 is a view similar to Figure 1 showing the piston blank after being machined. In general our invention deals withalloy compositions which in the cast condition prove unsatisfactory but which when used according to our disclosed process prove eminently satisfactory. The invention is specifically concerned, of course, with the method of making such pistons and with the product of metalloid silicon reduces the coefficient of ther- .nally introduced into the mixture.

ma] expansion of an aluminum alloy, it has been found commercially inexpedient to add more than about 25 per cent of silicon to the aluminum. Beyond this figure the primary silicon particles increase in number and size to such an extent that, particularly from a machining standpoint, the alloy cannot economically be handled. We have discovered, however, that by procuring comminuted mixtures of aluminum or magnesium with silicon or beryllium or combinations of these or other metals of any desired composition outside of the useful casting range, and compressing these mixtures in dies designed to produce either the approximate piston contour or such a convenient briquette that a piston may bemade therefrom, we secure a product which is characterized by the advantages of low specific gravity, low coefl'lcient of expansion; and other advantages as herein set forth.

For example, silicon may be used in the mixture of comminuted metallic powders in an amount substantially in excess of 25. per cent and uniformly divided into extremely small particles ofabout 200 mesh or finer. After a piston or briquette has been formed by compression of the powders the silicon particles disposed in the mixture will be no greater than the size origi- In casting, however, it is found that the size of the silicon crystals increases with an increase in the percentage of silicon in the alloy, and that when the silicon percentage approaches as much as 25 per cent the maximum size of the silicon crystals is substantially greater than 200 mesh. Furthermore; in chill casting pistons a uniform fine grained structure is obtained froman aluminum alloy containing no silicon or relatively small percentages of silicon, but if the silicon is increased to as much as 25 percent or more, the molten alloy will be relatively sluggish and this characteristic in connection with the tendency of the silicon to crystallize out in large. crystals hinders or prevents the proper filling of a mold. Furthermore, after the article is cast the presence of large silicon particles makes it diflicult, if not impossible, to machine the article smoothly and accurately. The silicon particles are relatively hard and cannot be cut by a cutting tool the alloying elements, and the specific gravity as easily as the softer aluminum matrix. The result is that the excessively large silicon 'particles tend to catch the cutting tool and roll or slide within the aluminum matrix leaving rough spots and depressions in the surface of the ar- 5 ticle. This difliculty is obviated by the method of the present invention, since the maximum size of the silicon particles can be initially predetermined regardless of thepercentage of the silicon introduced into the mixture. As a consequence articles may be produced in accordance with this invention containing silicon in excess of 25 per cent divided into particles, none of which exceed a size of about 200 mesh which may be machined as readily as aluminum castings containing only extremely small quantities of silicon and far more easily and accurately than castings produced from alloy containing substantial quantities of silicon approaching as much as 25 per cent. In this connection reference is made to the copending application of Louis W. Kempf et al., Serial No..615,028, filed June 2, 1932, now patented, No. 1,944,183, dated January 23, 1934, wherein certain specific alloys are claimed. 4 Although the alloying constituents-which we may use in addition to aluminum, magnesium, or beryllium with or without silicon, may be completely or only partially soluble in each other in the liquid state, and although failure of constituents to be soluble in each other in the liquid state is a practical bar to their use in any casting method, we are enabled by our method toform to any desired shape, and without depending on 'a casting method, any desired mixture of 'alloying constituents which, thoughthey are partially. soluble in each other in the liquid state, may not be completely so within the composition range which we desire to use for the obtainment .of the advantageous properties which are inher- 40 practicable to obtain a sound, dense'casting of an alloy of this composition although its coefficient of thermal expansion is only .000015 per degree centrigrade and its specific gravity is less even than that of pure aluminum, its desirability is instantly evident. This thermal expansion represents a reduction of more than per cent of the differential between iron and the ordinary aluminum base piston alloy, and a reduction of more than 50 per cent of the differential between iron and the low expansion alloy 60 disclosed in U. S. Patent 1,799,837 referred to hereinabove.

These figures illustrate the improvement effected in one case alone. Unlike the casting method, our process permits us to make a piston substantially as easily with any composition as with any other, and to choose any useful and desirable alloy constituent to any preferred amount. We can consequently prearrange the particle size of 0 and coefiicient of thermal expansivity of the final product.

Since low specific gravity is desirable in a piston we prefer to use-at least 25 per cent of aluminum or magnesium, or a combination of both to 7 5 For the manufacture of a piston of, 45

a minimum of about 25 per cent, with from at least 25 per cent up to a maximum of about 75 per cent of silicon by reason of its effect on the specific gravity and coemcient of thermal expansion of the combination. Beryllium may be added up to such amount as is economically practicable, since though at present fairly expensive its specific gravity and coefiicient-of thermal expansion are lower than either aluminum or magnesium. Lesser quantities of the heavier metals are contemplated for the practice of the invention since the principal object of the present invention is the combination of low specific gravity and coeflicient of expansion. For instance, we may use less than 20 per cent of nickel, iron, chromium, manganese, antimony, copper, molybdenum, titanum, tungsten or zinc. However, since the invention is directed specifically to a novel product and the process for producing it, the disclosure of specific compositions is in no sense limiting but is included as an indication of some useful combinations to which the invention may be directed.

With relatively high pressures the process as disclosed herein may be effected at room temperature but with pressures in the neighborhood of 50,000 pounds per square inch we prefer to preheat the dies to between-350C. and 450 C. At times it is even permissible to exceed to a slight extent, say 5 or 10 centigrade, the fusing temperature of the lowest melting point constituent of the'alloy composition of the mixture, but under ordinary conditions it is expected that the temperature of minimum fusibility will not be exceeded. In addition to 'quite satisfactory articles being thus produced, we' find that the homogeneity of the article may sometimes be improved by a final treatment at a fairly elevated temperature after fabrication. Using as an example an alloy of 60 per cent aluminum and 40 per cent silicon a favorable treatment is car- Test ried out at about 565 C. for 24 hours. bars from .alloys thus fabricated and heat treated and ofthe composition aluminum 70 per cent and silicon 30 per cent, have had a compression strength of 31,300 pounds per square inch after fabrication, and 'a compression strength of 47,300 pounds per square inch after heat treatment. This compares most favorably with the present commercial aluminum base piston alloys and-the alloy has a lower coeficient of thermal expansivity and a lower specific gravity, the other physical properties being adequate, for instance for service in an internal combustion engine piston.

Various mechanical means, may be used for the manufacture of pistons by my process. Dies to define the contour of a piston maybe made and the comminuted metal mixture forced into such dies with a hydraulic ram to produce a piston blank as shown in Figure 1'. Methods have recently been developed for upsetting pistons from solid metal and suchupsetting machines, with some alterations are adaptable to the present process. We have successfullyv produced blanks in a converted extrusion press provided with a plunger moving in a closed cylinder and we have machined the pistons to final dimensions to provide ring grooves and wrist pin bearings as illustrated in Figure 2 from these blanks. The weight of the finished article gives some indication of the weight of the metallic powders to be introduced into the dies.

We have machined a set of 6 pistons from blanks of the composition 60 per cent aluminum and 40 per cent silicon, an alloy which under service temperatures has a coefllcient of expansion of about .000015 per, degree centigrade. Using a straight trunk type piston and a substantiall-y'lower clearance than cou.d be used with 5 even the latest type low expansion aluminum base alloys heretofore available a block test was made on a commercial six cylinder motor. For the first 12 hours the motor speed was gradually inmine the particle'size of the various alloying con- 20 stituents which go to make up our piston. Although good results can be had with 40 mesh or finer we have found the best cohesion to take place with 90 mesh or finer as measured with the U. S. standard screen. ,We have. obtained very 25 excellent results with 1'70 mesh or finer. Themetal powder should be clean and oxides are to be particularly avoided. Since the granules of the various elements are mixed thoroughly and compressed into a coherent solid mass no trouble 3 isencountered with alloying elements which I might be immiscible or insoluble .in the molten or solid state if attempts were made to cast an article of the specific composition. Under high pressure and especially in the presence of heat the particles weld together toform a homogeneous whole. The coefilcient of expansion of the final article be proportional to the relative volume ofthe various constituents and the coeflicient of expansion of each constituent. In 40 this way any desired expansion effect can be attained.

Pistons made from comminuted metals in the manner herein described have an excellent hearing surface in relation to a ferrous metal cylin- 45 der. Their coeificient of expansion and specific gravity is limited only by the inherent characteristics' of the alloying materials used. Casting troubles are eliminated and with uniform temperature and pressure conditions. easily obtain- 50 able there should be only an insignificant number of rejections for imperfections. 1

In this specification and the appended claims the termpist0n blank" denotes a body compressed from comminuted metals, and being of 52 such adequate shape and size that a pistonmay be machined therefrom.

We claim:--

1. As a new article of manufacture a piston for 60 an internal combustion engine compressed from a comminuted mixture of metallic particles containing at least 25 per cent of aluminum and at least 25 per cent of silicon.

2. As a new article of manufacture a piston for an internal combustion engine compressed from a comminuted mixture of metallic particles containing at least 25 per cent of magnesium and at least 25 per cent of silicon. v

3. Asa new article of manufacture, a piston for an internal combustion engine composed of a light metal containing at least 25 per cent aluminum and at least 25 per cent of silicon, the silicon being interspersed through the metal and divided into substantially separated portions, the maximum size of said portions being the equivalent of about 200 mesh or liner.

4. As a new article of manufacture, a piston for an internal combustion engine consisting of a homogeneous, sound, uniform metallic structure containing at least 25 per cent of a metal of the group consisting of aluminum, magnesium and beryllium and at least 25 per cent of silicon, the

, silicon being divided into portions substantially aosmev minuted metallic powders including at least 25% of a light metal of the group consisting of aluminum, magnesium and beryllium and at least 25% of silicon in a die conforming approximately to the external contour of the piston, mechanically compressing said mixture under the influence of elevated temperatures into a blank of substantially the piston shape, and subsequently performing finishing operations on said blank.

7. A methodof'making a piston,-the method 10 consisting in compressing to form said blank 9. mixture of comminuted metallic powders under the influence of heat, said comminuted metallic powders including at least 25% aluminum and at least 25% of silicon, the silicon being interspersed.

through the mixture and divided into substantially separated portions, the maximum size of said portions beingabout 200 mesh or finer. 

